The structural stability of the co-chaperonin GroES has been studied by high sensitivity differential scanning calorimetry and circular dichroism under different solvent conditions. The thermal folding/unfolding of GroES is a spontaneous reversible process involving a highly cooperative transition between folded heptamers and unfolded monomers. During the denaturation process folded monomers are energetically unfavorable and consequently never become populated to an appreciable degree. Analysis of the high resolution structure indicates that isolated folded monomers of GroES bury a significantly smaller fraction of their total surface than typical globular proteins of similar molecular weight. For this reason the intramolecular interactions within each GroES monomer appear not to be sufficient for thermodynamic stabilization. The stabilization of the heptameric structure is due primarily to intersubunit interactions rather than intrasubunit interactions. These interactions favor oligomerization both enthalpically and entropically. Despite the high density of charged residues, the stability of GroES shows no measurable dependence on salt concentration at pH 7. On the other hand, millimolar concentrations of magnesium stabilize GroES, presumably by specific binding. The stabilization elicited by Mg2+ ion is consistent with a dissociation constant on the order of 0.5 mM and approximately three binding sites per heptamer. These results emphasize the role of quaternary structure in the stabilization of small oligomeric proteins.